Add Git Commit Reader

[mariugul]

Research on git wrappers like GitPython and pygit2 using libgit seems to introduce too much of a dependency. All I need is to read commits and not much more than that. Therefore, subprocessing git commands seems the best way.

This of course assumes git is present on the system. The git dependency should not be a problem, because who would want to lint their commits with comeit if they don't have git installed? 😄

Ideas

• Limit the number of commits to output. -n <number>, --max-count=<number>
• Show commits more recent than a specific date. --since=<date>, --after=<date>
• Show commits older than a specific date. --until=<date>, --before=<date>